Corrosion of metals is one of the main destruction processes of metallic structures leading to huge economic losses. Commonly, polymer coating systems are applied on the metal surface which provide a dense barrier for the corrosive species to protect metal structures from the corrosion attack. When the barrier is damaged and the corrosive agents penetrate to the metal surface the coating system is not able to stop the corrosion process. The most effective solution so far developed for anticorrosion coatings for active protection of metals is to employ chromate-containing conversion coatings. However, the hexavalent chromium species can be responsible for several diseases including DNA damage and cancer, which is the main reason for banning Cr6+-containing anticorrosion coatings in Europe from 2007.
The deposition of thin inorganic or hybrid films on metallic surface was suggested as a pre-treatment to provide an additional barrier for corrosion species and mainly to improve adhesion between metal at polymer coating system. The films are usually deposited by the plasma polymerization technique or the sol-gel route. Sol-gel derived thin films containing either inorganic (phosphates, vanadates, borates, cerium and molybdenum compounds) or organic (phenylphosphonic acid, mercaptobenzothiazole, mercaptobenzoimidazole, triazole) inhibitors were investigated to substitute chromates. Among them, the highest activity was shown for sol-gel coatings with cerium dopant of critical concentration in the 0.2-0.6 wt. % range. However, negative effects of the free inhibitor occluded in the sol-gel matrix on the stability of the protective film were observed for all types of inhibitors (for instance, higher concentration of Ce leads to the formation of microholes in the sol-gel film). In view of this shortcomings the use of reservoirs providing the corrosion inhibitor in isolated form inside the reservoir and preventing its direct interaction with sol-gel matrix was contemplated. Such reservoirs should be homogeneously distributed in the film matrix and should possess controlled and corrosion-stimulated inhibitor release to cure corrosion defects.
Mixed oxide nanoparticles (e.g., ZrO2—CeO2; M. L. Zheludkevich, R. Serra, M. F. Montemor, M. G. S. Ferreira, Electrochem. Commun. 2005, 8, 836), β-cyclodextrin-inhibitor complexes, (A. N. Khramov, N. N. Voevodin, V. N. Balbyshev, M. S. Donley, Thin Solid Films 2004, 447, 549), hollow polypropylene fibers (C. M. Dry, M. J. T. Corsaw, Cement and Concrete Research 1998, 28, 1133), conducting polyaniline (M. Kendig, M. Hon, L. Warren, Prog. Org. Coat. 2003, 47, 183) were explored as perspective reservoirs for corrosion inhibitors to be incorporated in the protective film. The common mechanism of such reservoir-based approaches is the slow release of inhibitor triggered by corrosion processes. Ion-exchangers were also investigated as “smart” reservoirs for corrosion inhibitors. Chemically synthesized hydrocalmite behaves as an anion exchanger to adsorb corrosive chloride ions releasing nitrite inhibiting anions (H. Tatematsu, T. Sasaki, Cement & Concrete Composites 2003, 25, 123).
Despite considerable efforts devoted to the development of new complex anticorrosion systems, practically no single solution is able to fulfill the requirements for sufficient corrosion protection avoiding chromates in the coating, especially in case of aluminium alloys used for aerospace applications. Approaches for combining inhibitor species and coating matrix, which were developed so far, have two considerable shortcomings: relatively short time of substrate protection, blistering and delamination of the coating thus degrading its physical and mechanical properties, destruction of barrier layer (often seen with molybdates and borates).
It would therefore be helpful to provide new, effective and broadly applicable means for providing active corrosion protection, in particular with self-healing ability.